The present invention relates generally to data communications. More particularly, the present invention relates to failover for aggregated data communication links.
Link aggregation or trunking is a method of combining multiple physical data communication links to form a single logical link, thereby increasing the capacity and availability of the communications channel between network devices such as servers, switches, end stations, and other network-enabled devices. For example, two or more Gigabit Ethernet or Fast Ethernet connections between two network devices can be combined to increase bandwidth capability and to create resilient and redundant links.
Link aggregation also provides load balancing, which is especially important for networks where it is difficult to predict the volume of data directed to each network device. Link aggregation distributes processing and communications activity evenly across a network so that no single network device is overwhelmed.
Link aggregation is documented in the Institute of Electrical and Electronics Engineers (IEEE) standard 802.3ad, which is incorporated by reference herein in its entirety. For convenience, several terms useful while discussing link aggregation are provided here.
Aggregator: A uniquely identifiable entity comprising (among other things) an arbitrary grouping of one or more aggregation ports for the purpose of aggregation. An instance of an aggregated link always occurs between exactly two Aggregators.
Aggregation Port: An instance of a Media Access Control-Physical Layer entity within an Aggregation System.
Aggregation System: A uniquely identifiable physical entity.
Aggregation Link: A data communication link that is an instance of a Media Access Control-Physical Layer-Medium-Physical Layer-Media Access Control entity between a pair of Aggregation Ports.
Aggregated Link: The logical link formed by the link aggregation of all of the aggregation links in an Aggregator.
Link aggregation provides several benefits, such as increased link availability. Link aggregation prevents the failure of any single aggregation link from causing a disruption of the communications between the interconnected network devices. While the loss of an aggregation link within an aggregated link reduces the available capacity of the aggregated link, the connection between the network devices is maintained and the data flow is not interrupted.
Link aggregation also increases link capacity. The performance of the communications between two network devices is improved because the capacity of an aggregated link is higher than the capacity of any of its constituent aggregation links. Link aggregation also permits data rates other than those that are generally available. Standard local-area network (LAN) technology provides data rates of 10 Mb/s, 100 Mb/s, and 1000 Mb/s. Link aggregation can fill the gaps between these available data rates when an intermediate performance level is more appropriate.
Link aggregation increases link availability and capacity without hardware upgrades. To increase link capacity, there are usually only two possibilities: either upgrading the native link capacity of the network devices, or aggregating two or more lower-speed aggregation links (if available). Upgrades are typically performed by increasing the link speed by an order of magnitude. In many cases, however, the network device cannot take advantage of this increase. A performance improvement of an order of magnitude is not achieved; moreover the bottleneck is simply moved from the network link to some other element within the network device. Thus, the performance is always limited by the end-to-end connection.
Link aggregation can be less expensive than a hardware speed upgrade and yet achieve a similar performance level. Both the hardware costs for a higher-speed link and the equivalent number of lower-speed connections have to be balanced to decide which approach is more advantageous. Sometimes link aggregation may even be the only means to improve performance when the highest data rate available on the market is not sufficient.
While link aggregation mitigates many problems, such as the failure of an aggregation link within an aggregated link, it fails to address other problems, such as switch failure.
FIG. 1 shows a data communications system 100 comprising a plurality of end stations 102A through 102N that communicate with a server 104A through another server 104B, a switch 106, and a plurality of aggregation links 108. Aggregation links 108 have been aggregated to form two aggregated links 110. Aggregated link 110A comprises aggregation links 108A, 108B, and 108C, which include respective aggregation ports P1, P2, and P3 in server 104A and respective aggregation ports P4, P5, and P6 in switch 106. Aggregated link 110B comprises aggregation links 108D, 108E, and 108F, which include respective aggregation ports P7, P8, and P9 in switch 106 and respective aggregation ports P12, P13, and P14 in server 104B.
Assume that each of aggregation links 108 is a Gigabit Ethernet link (that is, each of aggregation links 108 has a bandwidth of 1 Gb/s). Thus, each of aggregated links 110 has a bandwidth of 3 Gb/s. Should one of the aggregation links 108 within aggregated link 110A fail, aggregated link 110A would have a bandwidth of 2 Gb/s, and communications between servers 104A and 104B could continue at this reduced rate.
However, should switch 106 fail, communications between servers 104A and 104B would cease completely. Link aggregation cannot mitigate switch failure at all.